1. Field of the Invention
The present invention relates to a continuous dry drawing for a metal wire such as a steel wire and an apparatus therefor.
2. Description of the Related Art
A technique of drawing a metal wire to a specified diameter through dies is generally classified into a dry type method using a powder lubricant mainly containing metal soap and a wet type method using a liquid oil or water-soluble lubricant.
The important subject of the dry drawing method is the improvement of the drawing speed and the life of a die. To achieve this subject, lubrication and cooling techniques are required.
A cooling technique has been disclosed, for example in Examined Japanese Patent Publication No. SHO 51-31034.
As shown in FIG. 11, in drawing a metal wire 80 to a specified diameter through a plurality of dies 81, the prior art method includes the steps of removing scales on the surface of a metal wire in a pickling process; supplying the metal wire 80 subjected to a phosphate treatment from a supply stand; applying, on the surface of the metal wire, a dry lubricant 82 composed of solid metal soap or the like which is provided on the inlet side of each die; and passing the metal wire through each die 81; wherein a cooling unit 83 is provided on the downstream side of each die 81 for simultaneously cooling the die 81 and the metal wire 80 thereby improving the drawing speed.
A lubricating technique has been disclosed, for example in Examined Japanese Patent Publication No. SHO 45-9054, wherein a powder lubricant is pressed onto the surface of a wire by a roller to increase the adhesive force of the lubricant, thus improving the lubricating ability, and thereby enhancing the life of a die.
However, even by use of the above cooling technique combined with the above lubricating technique, there is a limitation in improving the drawing speed and the life of a die.
The limitation is mainly due to the lack of lubrication.
The diameter of a wire is reduced as the drawing proceeds, and the surface area per unit weight of the metal wire is increased linearly with the reduction ratio. Because of this increase in the surface area, the coating film formed in a drawing pre-treatment becomes thin. This coating film acts to enhance the picking-up of a dry lubricant, and accordingly, as the coating film becomes thin, the picking-up of the dry lubricant becomes insufficient, resulting in poor lubrication.
Also, part of the, coating film formed in the drawing pre-treatment is scraped as the metal wire passes through a plurality of dies, so that the picking-up of the dry lubricant becomes insufficient, resulting in poor lubrication.
Thus, when the total reduction ratio or the drawing speed is more than a specified value, the coating film is significantly deteriorated, which makes it difficult to perform the drawing.